Order of class initialization / instantiation

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Order of class initialization / instantiation

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#1 by (4 votes)

2

class Bird {
 { System.out.print("b1 "); }
 public Bird() { System.out.print("b2 "); }
}
class Raptor extends Bird {
 static { System.out.print("r1 "); }
 public Raptor() { System.out.print("r2 "); }
 { System.out.print("r3 "); }
 static { System.out.print("r4 "); }
}
class Hawk extends Raptor {
 public static void main(String[] args) {
 System.out.print("pre ");
 new Hawk();
 System.out.println("hawk ");
 }
}

The result displayed on the screen of the above code execution is:

r1 r4 pre b1 b2 r3 r2 hawk

However, I doubt that. I understand that since the Hawk class needs the Raptor class, but the Raptor class needs the Bird class, so I thought b1 should be the first to be displayed. Would it be possible to explain step by step how initializations happen and justify why?

java

asked by anonymous 18.03.2016 / 11:14

1 answer

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score 4 · Accepted Answer

In java, the boot order is as follows:

1- Runs all static components in the order they are   appear as soon as the class loads into the JVM

2- Runs all instance components in the order they are   appear in the class, so an object of this class is instantiated

3- Run the constructor.

So, first the members with the reserved word static will be executed, in the case of the informed class, were static { System.out.print("r1 "); } , static { System.out.print("r4 "); } and main itself, which is also static.

Then, the class instance members were executed, in the order they appear, in the case of the displayed code, only the { System.out.print("b1 "); } call.

Now it starts to be called the constructors, however, there is inheritance between the classes, which makes it "higher" (in this case the class Bird ) is the first to have its constructor, since the first thing a subclass does is call the constructor of its superclass, then first the constructor of Bird will be executed, after Raptor , finally the constructor of class Hawk will be executed, and only after that the System.out.println("hawk "); was displayed.

To better understand, I made this example:

public class TesteClasse {

    public static void main(String[] args) {
        new Filha();
    }
}

class Pai{

    static {System.out.println("Sou um membro estático da classe pai");}

    public Pai(){System.out.println("Sou o construtor da classe pai.");}

        {System.out.println("Sou um membro de estância da classe pai");}
}

class Filha extends Pai{

    {System.out.println("Sou um membro de estância da classe filha");}

    public Filha(){System.out.println("Sou  construtor da classe filha");}

static{System.out.println("Sou um membro estático da classe filha");} 

}

The output is:

Sou o método main
Sou um membro estático da classe pai
Sou um membro estático da classe filha
Sou um membro de estância da classe pai
Sou o construtor da classe pai.
Sou um membro de estância da classe filha
Sou  construtor da classe filha
Sou o método main de novo

See running on IDEONE .

References:

link